docs: def intersect_sites_method(form):
      """Return a method to intersect sites."""
      if settings.PAGE_USE_SITE_ID:
          if settings.PAGE_HIDE_SITES:
              site_ids = [global_settings.SITE_ID]
          else:
              site_ids = [int(x) for x in form.data.getlist('sites')]
          def intersects_sites(sibling):
              return sibling.sites.filter(id__in=site_ids).count() > 0
      else:
          def intersects_sites(sibling):
              return True
      return intersects_sites